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2009 MAGNETIC SYSTEMSY b 3+ → Er 3+ up-conversion luminescence under high pressure and pulsedmagnetic fieldsEr 3+ -doped materials are essential for laser application,displays, infrared detectors as well as in telecommunicationusing it in optical fibre amplifiers or other planar waveguides.Therefore, the understanding of the effect of an externalmagnetic field on the photoluminescence (PL) bandsof Er 3+ is important in order to obtain information aboutthe electronic structure, particularly, the Zeeman sublevelstructure forming the excited and ground state manifold.We have investigated the Zeeman splitting of the 4 S 3/2 → 4I 15/2 Er 3+ transition, which is responsible of the greenluminescence of high-quality single-crystal thin layers ofKY (WO 4 ) 2 : Er 3+ ,Y b 3+ in pulsed magnetic fields as afunction of hydrostatic pressure by resonant up-conversionspectroscopy (figure 102). The green Er 3+ PL was resonantlyexcited via up-conversion processes after Y b 3+ andEr 3+ excitation in the near infrared around 980 nm usinga tunable titanium-sapphire laser. Under the applied magneticfield, the energy resonance between the Y b 3+ levelsand the Er 3+ changes by the Zeeman effect. The upconversionprocess can be completely suppressed and therefore,a precise tuning of the laser excitation wavelength tocompensate the Zeeman splitting of Y b 3+ ions was necessaryin order to assure the resonance at high magnetic fields.Up-conversion process is a way to transform low energyphotons into higher energy photons different from secondharmonicgeneration which is commonly used in solid-statelasers. The most efficient up-conversion system is that onebased on Er 3+ and Y b 3+ . It is possible to observe, by thenaked eye, visible emission from Er 3+ after Y b 3+ excitationin the near-infrared.Figure 103: Pressure effect on the green photoluminescence of4 S 3/2 → 4 I 15/2 Er 3+ transition at two different external magneticfields.Figure 102: Low temperature Zeeman splitting of Er 3+ photoluminescencespectra in KY (WO 4 ) 2 : Er 3+ ,Y b 3+ for a 5.0 T magneticfield applied parallel to the crystallographic b axis correspondingto the lowest lying Kramers’ doublet of the 4 S 3/2 to the splitKramers’ doublets of the 4 I 15/2 ground state. The insets showdetails of some relevant peaks.The results indicate that pressure induces a linear redshiftfor all peaks at the two selected magnetic fields (figure 103).The pressure induced shift rates are almost independent ofthe magnetic field making this material suitable as sensor.In fact, the magnetic-field splitting mainly depends on thefield intensity and thus can be used as magnetic probes andthe highest energy peak position which depends on bothmagnetic field and pressure can be used in combinationwith the Zeeman splitting to unambiguously determine thepressure and magnetic field.Hence, KY (WO 4 ) 2 crystals doped with Y b 3+ and Er 3+ areexcellent systems for using as probes of high magneticfield and high pressure conditions through spectroscopicproperties of the Er 3+ green photoluminescence via upconversion.The PL spectrum at low temperature consistsof a series of Zeeman split peaks, which are distinctly sensitiveto the magnetic field intensity and pressure. This behaviorallows us to identify selected peaks by energy andintensity that provides an unambiguous determination of Band P simultaneously through the peak position E (B, P).The measured shift rates make it suitable for using as doublymagnetic and pressure sensor [Valiente et al., acceptedin High Pressure Research].M. Millot, S. George and J.M. BrotoR. Valiente, J. Gonzalez, F. Rodriguez (DCITIMAC, Santander, Spain), S. Garca-Revilla ( ESI, Bilbao, Spain), Y.Romanyuk (EMPA, Dübendorf, Switzerland) and M. Pollnau (MESA+, Enschede, Netherland)77